Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically comprise thin films of conductive, semiconductive and insulating materials that are patterned to form integrated circuits.
One type of semiconductor lithography involves placing a patterned mask between a semiconductor workpiece, and using an energy source to expose portions of a resist deposited on the workpiece, transferring the mask pattern to the resist. The resist is then developed, and the resist is used as a mask while exposed regions of a material on the workpiece are etched away.
As semiconductor devices are scaled down in size, lithography becomes more difficult, because light can function in unexpected and unpredictable ways when directed around small features of a mask. Several phenomenon of light can prevent the exact duplication of a mask pattern onto a wafer, such as diffraction, interference, or flare, as examples.
In many designs, the individual features of a circuit, such as gate lines or signal lines, as examples, have extremely small dimensions and may have widths of about 0.1 to 0.4 μm or less, with their lengths being considerably greater than the widths, e.g., about 0.8 to 2.0 μm or greater, for example. These thin lines may be connected to other layers of the integrated circuit by narrow vias filled with conductive material. When dimensions reach such a small size, there is not only a tendency for a line formed on a wafer to be shorter than its design length as defined by the lithography mask, but also the positioning of the vias may not be aligned to the target structures. Transfer differences of such critical dimensions occur when a desired circuit feature is particularly thin or small because of various optical effects. The accuracy of forming and positioning the lines and the vias becomes increasingly critical as dimensions decrease. Relatively minor errors in positioning such features can cause a via to miss the line altogether or to contact the line over a surface area that is insufficient to provide the necessary conductivity for a fully functional circuit. Thus, it is important to determine the effects of and to account for line shortening using optical measurements.
In the measurement of line shortening, gratings can be used to determine best focus and exposure dose, as described in an article entitled, “Focus Characterization Using End of Line Metrology,” by Leroux et al., in IEEE Transactions on Semiconductor Manufacturing, Vol. 13, No. 3, August 2000, pp. 322-330, and in an article entitled, “Distinguishing Dose From Defocus for In-Line Lithography Control,” by Ausschnitt, in Proc. SPIE, Vol. 3677, pp. 140-147, which are incorporated herein by reference. The ends of the thin lines of a grating structure are sensitive to focus, yet are “seen” as a solid line by an optical metrology tool using white light. The ends of lines appear as a solid line because the wavelength of the light is significantly larger than the printed features.
FIG. 1A shows a prior art sub-resolution grating pattern on a lithography mask. When imaged on a wafer or workpiece and viewed with white light on a wafer, an optical tool interprets the ends of the sub-resolution grating 209 lines as a solid line 207, as shown in FIG. 1B. However, when viewed with a scanning electron microscope (SEM), the gratings are visible, and the ends of the sub-resolution grating 209 lines appear as a single, somewhat ragged line, as shown in FIG. 1C. Because the lines and spaces of the grating on the mask 208 are smaller than the wavelength of light used in an optical microscope, the pattern formed on the workpiece is not visible by an optical tool, as shown in FIG. 1B. For example, the lines and spaces of the grating pattern 209 may be about 0.12 to 0.15 μm, and white light used in an optical microscope to view the workpiece may comprise a wavelength of about 650 nm or less, which is too large to resolve the pattern. However, a SEM has a higher resolution and can detect the pattern formed on the workpiece, and thus a SEM must be used to resolve the pattern on the workpiece.
Though an optical alignment tool can measure the relative line shortening, the measurements are sensitive to pitch and contrast, as described in an article entitled, “Understanding Optical End of Line Metrology,” by Ziger, et al., in Opt. Eng., July 2000, Vol. 39, No. 7, pp. 1951-1957, which is incorporated herein by reference. For certain applications, a relative measurement is all that is required to measure line shortening. As an example, optimum focus can be determined from the minimum line shortening a modified box-in-box structure, as described in the article “Focus Characterization Using End of Line Metrology,” previously referenced herein. However, the actual line shortening and line shortening optically measured are quite different due to diffraction effects.
There are applications using gratings that are read with optical tools, for which an absolute value of line shortening is desirable. For example, in U.S. Pat. No. 5,962,173, issued on Oct. 9, 1999, entitled, “Method for Measuring the Effectiveness of Optical Proximity Corrections,” which is incorporated herein by reference, Leroux, et al. describe a method for measuring optical proximity effect using a modified box in box structure that contains gratings which are read using an optical measuring instrument. This method requires calibration of the optical metrology tool to ensure that the measurements are correct.
In U.S. Pat. No. 6,301,008, issued on Oct. 9, 2001, entitled, “Arrangement and Method for Calibrating Optical Line Shortening Measurements,” which is also incorporated herein by reference, Ziger et al. describe an approach to calibrate optical and SEM-based measurements. However, this approach requires a correlation between actual and optical measurements as a function of pitch and line size.
A SEM cannot be used to make measurements of line shortening, because the gratings of the test patterns are too long for the SEM to measure. A SEM is useful for high magnification measurements, but a SEM cannot measure the line length of a long line. For example, if there is line shortening of 0.1 μm in a test pattern grating having a length of 9 μm, a SEM cannot be used to measure the line shortening, and thus a SEM cannot be used to calibrate optical metrology tools for such a test pattern.
What is needed in the art are improved methods of calibrating line shortening measurements of optical measurement or metrology tools in semiconductor lithography systems.